Răspuns :
⇔ x²+2x-(6x²+8x+9x+12) = 18-5x²
⇔ x²+2x-6x²-8x-9x-12 = 18-5x²
⇔ x²+2x-6x²-8x-9x+5x² = 18+12
⇔ -15x = 30
⇔ x= - 2
⇔ x²+2x-6x²-8x-9x-12 = 18-5x²
⇔ x²+2x-6x²-8x-9x+5x² = 18+12
⇔ -15x = 30
⇔ x= - 2